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MYRTLE BEACH, S.C., Jan. 21 -- The clash between Barack Obama and Hillary Rodham Clinton Monday night was a debate long waiting to happen, and at the heart of it was the man who was not on stage: former president Bill Clinton.

Since Obama's victory in Iowa, the Clintons have responded with a methodically aggressive campaign. With his own campaign now on the defensive, Obama came to Monday's debate determined to confront his principal rival for the Democratic presidential nomination with a cry of foul.

The result was the most heated and acrimonious exchange of the long race. The opening minutes included charges and countercharges, personal attacks and some of the harshest language either of the two leading Democratic candidates has used, particularly face to face.

So intense was their dispute that former senator John Edwards (N.C.) repeatedly demanded the opportunity to be heard.

Democratic strategists differed on whether Clinton or Obama emerged as the winner of the two-hour session. Some said Obama came off at points as defensive and working too hard to explain his record and his positions. Others said Clinton was so aggressive that it could cost her support. Edwards's backers hope that he will benefit if voters are turned off by his rivals' tone and temperament.

The encounter had two audiences: voters in South Carolina, a state Obama must win and is expected to win, largely on the strength of his solid support in the African American community, and voters in the 22 states with primaries or caucuses on Feb. 5. Clinton is concentrating her efforts there in hopes of gaining a decisive advantage.

Obama has learned how formidable the Clintons' political machine can be, particularly when its future is on the line. The former president has played the lead role in taking the fight to the senator from Illinois. Obama and his advisers have been seething about what both Clintons -- but particularly the former president -- have said about him over the past two weeks.

Coupled with Hillary Clinton's surprise victory in New Hampshire two weeks ago and her bruising win in the Nevada caucus last Saturday, the comments have raised frustrations in the Obama campaign to the boiling point.

Relations between Obama and Clinton have been chilly from the start of this contest, but their hostility has intensified as the primary-caucus season has opened. It was unmistakable on Monday.

At one point, Obama, seeking to demonstrate his long commitment to displaced workers, said he had been working with them on the streets of Chicago at a time when Clinton was on the corporate board of Wal-Mart. "I was fighting these fights," he said.

Clinton quickly retaliated: "I was fighting against those [Republican] ideas when you were practicing law and representing your contributor . . . in his slum landlord business in inner-city Chicago."